The Application relates to new aromatic formals, thermoplastic molding compositions comprising polycarbonate and at least one aromatic formal according to the invention as an additive for lowering the water uptake of the polycarbonate and for improving the flowability, and the use of such molding compositions for the production of molded articles, in particular optical data carriers, such as e.g. compact discs, video discs, digital versatile discs and further optical data carriers which may be written to and deleted once or several times, and the corresponding molded articles themselves.
Polycarbonates are employed generally, because of their particular combination of properties, such as transparency, heat resistance and dimensional stability, as materials for injection molding or injection-compression molding of optical data carriers. To improve the processability, processing in general taking place at temperatures in the range from 300° C. to 400° C., additives such as mold release agents and stabilizers are as a rule added to the polycarbonate.
Aromatic polycarbonates based on bisphenol A are used in particular for the production of optical data carriers. However, they may absorb up to 0.34 wt. % of water, which may have an adverse effect on the dimensional stability of the data carriers. An improved dimensional stability is of importance, however, especially if blue or blue-green lasers are employed.
U.S. Pat. No. 6,391,418 describes substrates for data carrier media which comprise a biphenyl derivative as an additive for increasing the dimensional stability (lower shrinkage).
The addition of small amounts of m-terphenyl to bisphenol A polycarbonate, which leads to a reduction in the water uptake, is described in M. Ueda, Mitsubishi Engineering Plastics Corp., Technical Digest of Joint ISOM/ODS 2002 Waikoloa Hi., 8.7.2002, pages 33-35. The disadvantage of these biphenyl derivatives, however, is that they are highly conjugated aromatic π systems which already absorb in the blue or blue-green spectral region. This is undesirable in storage technologies which operate in this wavelength range. Furthermore, terphenyls are relatively rigid molecules, which has an adverse effect on the mechanical properties in the mixture with polycarbonate.
The possibilities described in the prior art thus do not lead to satisfactory results in every respect. However, no indication at all that formals could be suitable as additives is to be found in the prior art.
There was therefore the object of providing thermoplastic molding compositions which comprise polycarbonate with a reduced water uptake and as a result have a better dimensional stability. In particular, the new disc formats with a relatively high storage capacity and-possibly a thinner disc thickness, such as e.g. digital versatile discs (DVDs), require a higher heat stability compared with the CD. Damage to the material occurring during processing to molded articles and the formation of a deposit in the mold become more critical. It is thus desirable that an additive for reducing the water uptake at the same time has the effect of a lowering of the melt viscosity and therefore a better flow at somewhat lower temperatures.
With the molding compositions according to the invention, this object is surprisingly achieved by an improved quality of the data storage medium and an improved processability of the material in injection molding or the injection-compression molding process and a reduced water uptake and therefore improved dimensional stability.
The present Application therefore provides thermoplastic molding compositions comprising at least one polycarbonate and at least one aromatic formal according to the invention with a specific chemical structure as an additive for reducing the water uptake. These aromatic formals thus lead to an improved dimensional stability of the data carriers and at the same time have the effect of a lower melt viscosity.
In contrast to polycarbonate, aromatic polyformals may be prepared in a homogeneous phase from bisphenols and methylene chloride in the presence of alkali metal hydroxides. In this polycondensation, methylene chloride functions simultaneously as a reactant and as a solvent. U.S. Pat. No. 4,374,974 describes a process in which, starting from specific bisphenols, linear and cyclic oligo- and polyformals may be obtained after reaction with methylene chloride. The conversion of monofunctional phenols to low molecular weight aromatic formals by means of this synthesis variant is not described in the prior art.
S. Tanimoto et al., Bull. Inst. Chem. Res., Kyoto Univ., vol. 56, no. 6, 1978 discloses a synthesis in DMSO or acetonitrile in the presence of 18-crown-6 for certain methyl-, chloro-, bromo- and methoxy-substituted species as a new possible use for this phase transfer catalyst. However, nothing is said about the synthesis of further formals or the usability thereof.